Gas sensor using porous nano-fiber containing metal oxide and manufaturing method thereof

ABSTRACT

Disclosed is a method of manufacturing a gas sensor by using a nano-fiber including metal oxide. The method of manufacturing the gas sensor includes the steps of (1) mixing a polymer precursor with a solvent, (2) dispersing metal oxide into the mixture obtained through step (1), (3) preparing a nano-fiber by performing electro-spinning with respect to the mixture obtained through step (2), (4) oxidizing the nano-fiber obtained through step (3), (5) carbonizing the nano-fiber that has been oxidized through step (4), (6) activating the nano-fiber that has been carbonized through step (5), and (7) manufacturing the gas sensor by depositing the nano-fiber, which has been activated through step (6), between electrodes of a silicon wafer. The gas sensor is manufactured with superior sensitivity at a normal temperature and reliability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas sensor. More particularly, thepresent invention relates to a method of manufacturing a gas sensor,which can represent reliability with remarkably high sensitivity at anormal temperature, by using a porous nano-fiber including metal oxide,and the gas sensor manufactured through the method.

2. Description of the Prior Art

In general, a gas sensor measures the quantity of noxious gas based on acharacteristic in which the electrical conductivity varies according tothe adsorption of gas molecules. Materials mainly used in the gas sensorinclude a metal oxide semiconductor such as SnO₂, a solid electrolytematerial, various organic materials, and the complex of carbon black andan organic material. The gas sensor manufactured by using the abovematerials has many problems such as restricted use. In other words, agas sensor manufactured by using the metal oxide semiconductor or thesolid electrolyte material has to be heated at the temperature of about200° C. to 600° C., or more in order to perform the normal operation. Agas sensor manufactured by using the organic material representsextremely low electrical conductivity, and a gas sensor manufactured byusing the complex of the carbon black and the organic materialrepresents significantly low sensitivity. In addition, the conventionalgas sensors manufactured by the above materials requires long time forsensing, represents remarkably low recovery speed, and requires a highprice, so that the conventional gas sensors are unsuitable for generalpurposes.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problem occurringin the prior art, and an object of the present invention is to provide agas sensor and a method of manufacturing the same, capable ofrepresenting remarkably high sensitivity at a normal temperature bypreparing a nano-fiber including metal oxide through electro-spinning,and depositing the nano-fiber between silicon wafer electrodes afteroxidizing, carbonizing, and activating the nano-fiber.

In order to accomplish the object, there is provided a method ofmanufacturing a gas sensor by using a nano-fiber including metal oxide.The method of manufacturing the gas sensor includes the steps of (1)mixing a polymer precursor with a solvent, (2) dispersing metal oxideinto the mixture obtained through step (1), (3) preparing a nano-fiberby performing electro-spinning with respect to the mixture obtainedthrough step (2), (4) oxidizing the nano-fiber obtained through step(3), (5) carbonizing the nano-fiber that has been oxidized through step(4), (6) activating the nano-fiber that has been carbonized through step(5), and (7) manufacturing the gas sensor by depositing the nano-fiber,which has been activated through step (6), between electrodes of asilicon wafer.

Preferably, the method further includes performing heat treatment withrespect to the gas sensor, which has been obtained through step (7),after step (7) has been performed. Preferably, the heat treatment isperformed at a temperature of 30° C. to 80° C. for 0.1 to one hour.

Preferably, the mixture, which has been obtained through step (2), hasviscosity in the range of 100 cP to 500 cP.

Preferably, in step (2), 2 to 10 parts by weight of the metal oxide isdispersed into the mixture, which has been obtained through step (1),based on 100 parts by weight of the mixture. The activation degree ofthe nano-fiber is increased by adding the metal oxide serving as acatalyst, thereby manufacturing the gas sensor having superiorsensitivity and rapid response.

Preferably, in step (4), the nano-fiber is oxidized while raising atemperature at a rate of 1° C./min to 5° C./min, and oxidized at atemperature of 200° C. to 300° C. for two hours to five hours in a finalstage.

Preferably, in step (5), the nano-fiber is carbonized while raising atemperature at a rate of 5° C./min to 10° C./min, and carbonized at thetemperature of 800° C. to 1200° C. for a half an hour to two hours in afinal stage.

Preferably, the activation in step (6) of the nano-fiber is achieved byapplying a potassium hydroxide solution, and the potassium hydroxidesolution has density in a range of 5 M to 10 M.

Preferably, in step (7), the gas sensor is manufactured by dispersingthe nano-fiber, which has been obtained through step (6), into adispersion solution and depositing the nano-fiber between electrodes ofa silicon wafer, and a ratio of the nano-fiber dispersed into thedispersion solution is in a range of 0.1 to 3 parts by weight based on100 parts by weight of the dispersion solution.

In addition, the present invention provides a gas sensor manufacturedthrough the method.

As described above, the gas sensor capable of representing remarkablyhigh sensitivity at a normal temperature can be manufactured bypreparing a nano-fiber including metal oxide through electro-spinning,and depositing the nano-fiber between silicon wafer electrodes afteroxidizing, carbonizing, and activating the nano-fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a gas sensor manufactured accordingto an embodiment of the present invention;

FIG. 2 is a schematic view showing electro-spinning equipment used toprepare a nano-fiber;

FIG. 3 is a view showing nitrogen adsorption isotherms used to determinethe porosity of nano-fibers prepared according to embodiment 1 andcomparative example 1 of the present invention;

FIG. 4 is an SEM image taken to examine the surface characteristic ofthe gas sensor manufactured according to the embodiment of the presentinvention;

FIG. 5 is a schematic view showing a device to measure a gas sensingcharacteristic; and

FIG. 6 is a graph showing a sensing characteristic for NO gas of gassensors prepared according embodiment 2 and comparative example 2 of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a gas sensor manufactured by using anano-fiber including metal oxide and a method of manufacturing the same.FIG. 1 is a schematic view showing a gas sensor manufactured accordingto an embodiment of the present invention. As shown in FIG. 1, in thegas sensor according to the present invention, porous nano-fibersincluding metal oxide are uniformly distributed between electrodes of asilicon wafer.

Hereinafter, the present invention will be described in detail.

The present invention provides the method of manufacturing the gassensor by using the porous nano-fibers including metal oxide. The methodof manufacturing the gas sensor includes the steps of (1) mixing apolymer precursor with a solvent, (2) dispersing metal oxide into themixture obtained through the step (1), (3) preparing a nano-fiber byperforming electro-spinning with respect to the mixture obtained throughthe step (2), (4) oxidizing the nano-fiber obtained through the step(3), (5) carbonizing the nano-fiber oxidized through the step (4), (6)activating the nano-fiber carbonized through the step (5), and (7)manufacturing the gas sensor by depositing the nano-fiber, which hasbeen activated through the step (6), between electrodes of the siliconwafer.

The polymer precursor used in the step (1) may include various materialsthat can be changed into carbon. In detail, the polymer precursor may beselected from the group consisting of petroleum pitch, coal pitch,polyimide, polybenzimidazol, polyacrylonitrile, polyaramid, polyaniline,mesophase pitch, furfuryl alcohol, phenole, cellulose, sucrose, poly(vinyl chloride), and the mixture thereof.

The solvent used in the step (1) may include various solvents capable ofdissolving the polymer precursor. For example, the solvent may beselected from the group consisting of dimethylformamide, chloroform,N-methylpyrrolidonetetrahydrofuran, sulfuric acid, nitric acid, aceticacid, hydrochloric acid, ammonia, distilled water, and the mixturethereof.

The metal oxide used in the step (2) is not limited to a specific type,but generally-known metal oxide may be used.

The viscosity of the mixture obtained in the step (2) is preferably inthe range of about 100 cP to about 500 cP. The mixture, which has beenobtained in the step (2), is subject to the electro-spinning process inthe next step by using electro-spinning equipment shown in the schematicview of FIG. 2. The electro-spinning equipment is typicalelectro-spinning equipment. Hereinafter, the operation of theelectro-spinning equipment will be briefly described. As shown in FIG.2, the electro-spinning equipment includes a metering pump 1, a voltagegenerator 2, a concentrator 3, and a radiator 4. First, a solution isinjected into the radiator 4 through the metering pump 1, and thesolution radiated through the radiator 4 is concentrated by theconcentrator that is rotating. The voltage generator 2 applies a desiredvoltage. In other words, the mixture obtained in the step (2) isradiated through the radiator 4. However, if the viscosity of themixture exceeds 500 cP, the nozzle of the radiator 4 is clogged due tothe cohesive force between polymer molecules, so that the solution isnot smoothly radiated. In addition, if the viscosity of the mixture isless than 100 cP, the polymers do not form a predetermined shape due tothe significantly low viscosity.

When dispersing metal oxide into the mixture, which has been obtainedthrough the step (1), in the step (2), 2 to 10 parts by weight of themetal oxide is preferably mixed with the mixture based on 100 parts byweight of the mixture. If less than 2 parts by weight of the metal oxideis mixed, the activation degree of the nano-fiber is not changed whencomparing with a case in which the metal oxide is not mixed.Accordingly, the less than 2 parts by weight of the metal oxide is notproper. If more than 10 parts by weight of the metal oxide is mixed, themetal oxide is difficult to be dispersed into the mixture obtained inthe step (1). Accordingly, more than 10 parts by weight of the metaloxide is not proper.

In order to prepare the nano-fiber by performing the electro-spinningwith respect to the mixture, which has been obtained through the step(2), in the step (3), a typical electro-spinning process is performed byusing the electro-spinning equipment shown in FIG. 2.

When oxidizing the nano-fiber in step (4), preferably, the nano-fiber isoxidized while raising a temperature at a rate of 1° C./min to 5°C./min, and oxidized at the temperature of 200° C. to 300° C. for twohours to five hours in the final stage.

If the nano-fiber is oxidized while raising a temperature at a rate oflower than 1° C./min, cyclization reaction caused by the reaction ofoxygen and carbon may not smoothly occur due to the lower reaction rate,and a fiber yield rate may be degraded. If the nano-fiber is oxidizedwhile raising a temperature at a rate of more than 5° C./min, thenano-fiber is unstably formed due to the fast reaction rate, so that thenano-fiber may be melted or subject to glass transition in thecarbonizing step that is the next step. As a result, the nano-fibercannot maintain a fiber form. If the oxidization temperature is lessthan 200° C., the oxidation reaction may unstably occur, so that thenano-fiber may be melted or subject to glass transition in thecarbonizing step that is the next step, and the nano-fiber cannotmaintain a fiber form. In addition, condensation reaction between carbonatoms is not smoothly achieved. In addition, if the oxidizationtemperature exceeds 300° C., the reaction may be rapidly induced due tothe high temperature, so that cyclization a carbon-oxygen bond reactionmay not smoothly occur due to in an excessive oxygen state. If anoxidizing process is preformed for less than two hours, there occurs aphenomenon the same as that of a case in which the final oxidizationtemperature is less than 200° C. In addition, if the oxidization timeexceeds five hours, there is no difference from when the oxidation isperformed for five hours, and undesirable reaction may occur.

When carbonizing the nano-fiber in step (5), the nano-fiber iscarbonized while raising a temperature at a rate of 5° C./min to 10°C./min, and carbonized at the temperature of 800° C. to 1200° C. for ahalf an hour to two hours in the final stage. If the nano-fiber iscarbonized at a rate of lower than 5° C./min and if the nano-fiber iscarbonized at a temperature exceeding 1200° C., reaction time isprolonged and a great amount of energy is consumed. If the temperatureis raised at a rate of more than 10° C./min, volatilizationsignificantly occurs, so that the yield rate of the nano-fiber may belowered. If the nano-fiber is carbonized at a temperature of lower than800° C., the carbonization may not occur completely. If the nano-fiberis carbonized for less than a half an hour, the nano-fiber may beinsufficiently carbonized. If the nano-fiber is carbonized for more thanfive hours, there is no difference from when the nano-fiber iscarbonized for five hours, and insufficient reaction may occur.

In the step (6), the activation of the nano-fiber is achieved byapplying a potassium hydroxide solution, and the density of thepotassium hydroxide solution is preferably in the range of 5 M to 10 M.If the density of the potassium hydroxide solution is less than 5 M, theactivation may not occur sufficiently. If the density of the potassiumhydroxide solution exceeds 10M, there is no difference from when thedensity of the potassium hydroxide solution is identical to 10M, so thatthe increase of the density of the potassium hydroxide to more than 10Mhas no special benefit in practice.

When manufacturing the gas sensor in step (7), the nano-fiber, which hasbeen activated through the step (6), is dispersed into a dispersionsolution, and deposited between electrodes of a silicon wafer. Varioustypes of dispersion solutions may be used. For example, the dispersionsolution may be selected from the group consisting of ethanol, methanol,acetone, dimethylformamide, and the mixture thereof. The ratio of thenano-fiber dispersed into the dispersion solution is preferably in therange of about 0.1 to 3 parts by weight based on 100 parts by weight ofthe dispersion solution. If less than 0.1 parts by weight of thenano-fiber is dispersed, the nano-fiber may be not uniformly distributedbetween the electrodes of the silicon wafer. If more than 3 parts byweight of the nano-fiber is dispersed, the nano-fiber is difficult to bedispersed in the dispersion solution. Hereinafter, the present step ofmanufacturing the gas sensor will be further described. After finelygrinding the nano-fiber obtained through the step (6), the nano-fiber isuniformly dispersed in a dispersion solution at a desired ratio. Next,the solution having the dispersed nano-fiber is deposited on the siliconwafer in which a para film is attached to a wire connection part. Thedeposition may be performed through a vapor deposition scheme, a spincoating scheme, and a spray deposition scheme.

In addition, the present invention may further include a step ofperforming heat treatment with respect to the gas sensor, which has beenobtained through the step (7), after the step (7) has been performed.The heat treatment is preferably performed at a temperature of about 30°C. to 80° C. for about 0.1 to one hour. If the temperature of the heattreatment is less than 30° C., the dispersion solution may be not easilyevaporated. If the temperature of the heat treatment exceeds 80° C., apara film used to protect a connection part of a wire may be melted. Ifthe time for the heat treatment is less than 0.1 hour, the dispersionsolution may be not easily evaporated. If the time for the heattreatment exceeds one hour, effects are represented similarly to thoseof a case in which the time for the heat treatment is 1 hour.

In addition, the present invention provides a gas sensor manufacturedthrough the method.

Hereinafter, the present invention will be described in more detailthrough embodiments.

Embodiment 1 Prepare of Porous Nano-Fiber Including Metal Oxide (ZincOxide)

Polyacrylonitrile was dissolved in dimethylformamide, so that the mixedsolution was manufactured. About 6.7 parts by weight of zinc oxide (ZnO)based on 100 parts by weight of the mixed solution is dispersed in themixed solution. Through the process, the viscosity of the mixed solutionin which the ZnO was dispersed was adjusted to about 160 cP.

The nano-fiber was manufactured by electro-spinning the mixed solution.The electro-spinning must be performed under a condition of a voltage of17 kV, the distance (TCD) of about 12 cm between the concentrator andthe tip of an injector, a pump flow rate of 2.0 ml/h, and the rotationrate of the concentrator of about 200 rpm.

The nano-fiber manufactured through the electro-spinning was oxidizedwhile raising a temperature at a rate of 2° C./min. In the final stage,the nano-fiber is oxidized at a temperature of 250° C. for three hours.

The nano-fiber, which had been oxidized, was carbonized while raising atemperature at a rate of 7° C./min. In the final stage, the nano-fiberwas carbonized at a temperature of 1050° C. for one hour. In the processof carbonizing the nano-fiber, nitrogen (N₂) gas was injected at a flowrate of about 20 cc/min.

The nano-fiber, which had been carbonized, was dipped into 8M of apotassium hydroxide solution, shaken for on hour, and activated. Theactivation process for the nano-fiber was performed at a temperature ofabout 750° C. for three hours.

Comparative Example 1 Prepare of Nano-Fiber

Poly acrylonitrile was dissolved in dimethylformamide until the mixedsolution of the Poly acrylonitrile and the dimethylformamide has theviscosity of 160 cP, so that the mixed solution was prepared. Thesubsequent processes of preparing the Nano-fiber are identical to thoseof Embodiment 1.

Analysis of Specific Surface Characteristic

Specific surface characteristics of the nano-fibers according toembodiment 1 and comparative example 1 were analyzed and the resultthereof was shown in FIG. 3. The nano-fiber according to embodiment 1has the specific surface of about 1.305 m²/g, and the surface area ofthe nano-fiber having fine pores is 62% based on the specific surface.The nano-fiber according to comparative example 1 has the specificsurface of about 1.741 m²/g, and the surface area of the nano-fiberhaving fine pores is 64% based on the specific surface. When comparingwith comparative example 1, the nano-fiber according to embodiment 1 isreduced by about 25% in the specific surface, and reduced by about 3% inthe surface area made by fine pores.

Embodiment 2 Manufacture of Gas Sensor Using Porous Nano-Fiber IncludingZnO

The nano-fiber manufactured according to embodiment 1 was crushed intofine powder by using a mortar, and the powder of the nano-fiber wasdispersed into dimethylformamide. In this dispersion process, 2 parts byweight of the powder of the nano-fiber was dispersed based on 100 partsby weight of dimethylformamide.

The mixed solution prepared through the above processes was dropped ontothe silicon wafer, and then spin-coating was performed at a rotationalrate of about 900 rpm for 4 minutes, thereby manufacturing the gassensor.

In the final sage, the gas sensor deposited with the nano-fiber, whichhad been manufactured through the above process, was provided on a hotplate and heat-treated at a temperature of 40 for 0.5 hours.

Comparative Example 2 Manufacture of Gas Sensor Using Nano-Fiber

The nano-fiber manufactured according to comparative example 1 wascrushed into fine powder by using a mortar, and the powder of thenano-fiber was dispersed into dimethylformamide. In this dispersionprocess, 2 parts by weight of the powder of the nano-fiber was dispersedbased on 100 parts by weight of dimethylformamide. The subsequentprocesses of manufacturing the gas sensor are identical to those ofEmbodiment 2.

Surface Characteristic

In order to observe the surface characteristic of the gas sensormanufactured according to embodiment 2, a picture of the gas sensor wastaken by using an SEM (Scanning Electron Microscope) and shown in FIG.4. As shown in FIG. 4, the porous nano-fiber including ZnO was uniformlydeposited on the silicon wafer.

Gas Sensing Characteristic

Hereinafter, the measurement and the estimation of the gas sensingcharacteristics of the gas sensors manufactured embodiment 2 andcomparative example 2 will be described. FIG. 5 is a schematic viewshowing an apparatus to estimate the gas sensing characteristics. Beforeestimating the gas sensing characteristics, pre-processes for the gassensors were performed at a temperature of 80° C. for 0.5 hours toevaporate the moisture of the gas sensors before the gas sensingcharacteristics were estimated. The gas sensing characteristics weremeasured by injecting NO gas with the density of 50 ppm at a temperatureof 25° C. (normal temperature) by using the apparatus shown in FIG. 5,and the measurement result is shown in FIG. 6. As shown in FIG. 6, theresistance change ratio of the gas sensor according to embodiment 2 isabout 13.5% in which the resistance change ratio represents thesensitivity of the gas sensor according to the sensing characteristicfor the NO gas, and the resistance change ratio of the gas sensoraccording to comparative example 2 is about −9%.

In the graph shown in FIG. 6, an X axis represents a measurement time,and a Y axis represents the resistance change ratio. As shown in FIG. 6,the resistance change ratio of the gas sensor according to embodiment 2at the N2 gas atmosphere is stabilized after about 4.5 minutes to about5 minutes had been elapsed. In addition, time of about 4.5 minutes toabout 5 minutes was required until the resistance change ratio reached alimited value at the atmosphere of about 50 ppm of NO. In contrast, theresistance change ratio of the gas sensor according to comparativeexample 2 at the N2 gas atmosphere is stabilized after about 5 minutesto about 6 minutes had been elapsed. In addition, time of about 6minutes to about 7 minutes was required until the resistance changeratio reached a limited value at the atmosphere of about 50 ppm of NO.

Although the gas sensor according to the embodiment of the presentinvention has the specific surface smaller than that of the gas sensoraccording to the comparative example, the gas sensor according to theembodiment of the present invention can represent the higher resistancechange ratio and the shorter response time at a normal temperature. Thisis because the metal oxide constituting the gas sensor according to theembodiment of the present invention serves as a catalyst, so that theactivation of the nano-fiber can be increased.

Therefore, the gas sensor according to the embodiment of the presentinvention can represent the higher resistance change ratio and theshorter response time at a normal temperature. In other words, the gassensor according to the embodiment of the present invention has highersensitivity at a normal temperature.

Although the exemplary embodiments of the present invention have beendescribed, it is understood that the present invention should not belimited to these exemplary embodiments but various changes andmodifications can be made by one ordinarily skilled in the art withinthe spirit and scope of the present invention as hereinafter claimed.

1. A method of manufacturing a gas sensor comprising: (1) mixing apolymer precursor with a solvent; (2) dispersing metal oxide into themixture obtained through step (1); (3) preparing a nano-fiber byperforming electro-spinning with respect to the mixture obtained throughstep (2); (4) oxidizing the nano-fiber obtained through step (3); (5)carbonizing the nano-fiber that has been oxidized through step (4); (6)activating the nano-fiber that has been carbonized through step (5); and(7) manufacturing the gas sensor by depositing the nano-fiber, which hasbeen activated through step (6), between electrodes of a silicon wafer.2. The method of claim 1, further comprising performing heat treatmentwith respect to the gas sensor, which has been obtained through step(7), after step (7) has been performed.
 3. The method of claim 1,wherein the mixture, which has been obtained through step (2), hasviscosity in the range of 100 cP to 500 cP.
 4. The method of claim 1,wherein, in step (2), 2 to 10 parts by weight of the metal oxide isdispersed into the mixture, which has been obtained through step (1),based on 100 parts by weight of the mixture.
 5. The method of claim 1,wherein, in step (4), the nano-fiber is oxidized while raising atemperature at a rate of 1° C./min to 5° C./min, and oxidized at atemperature of 200° C. to 300° C. for two hours to five hours in a finalstage.
 6. The method of claim 1, wherein, in step (5), the nano-fiber iscarbonized while raising a temperature at a rate of 5° C./min to 10°C./min, and carbonized at the temperature of 800° C. to 1200° C. for ahalf an hour to two hours in a final stage.
 7. The method of claim 1,wherein the activation in step (6) of the nano-fiber is achieved byapplying a potassium hydroxide solution.
 8. The method of claim 7,wherein the potassium hydroxide solution has density in a range of 5 Mto 10 M.
 9. The method of claim 1, wherein, in step (7), the gas sensoris manufactured by dispersing the nano-fiber, which has been obtainedthrough step (6), into a dispersion solution and depositing thenano-fiber between electrodes of a silicon wafer.
 10. The method ofclaim 9, wherein the dispersion solution is selected from the groupconsisting of ethanol, methanol, acetone, dimethylformamide, and themixture thereof.
 11. The method of claim 9, wherein a ratio of thenano-fiber dispersed into the dispersion solution is in a range of 0.1to 3 parts by weight based on 100 parts by weight of the dispersionsolution.
 12. The method of claim 2, wherein the heat treatment isperformed at a temperature of 30° C. to 80° C. for 0.1 to one hour. 13.(canceled)